The use of steam to enhance the production of heavy oil from heavy oil bearing formations is known. Heavy oil is highly viscous at room temperatures which makes easy flow of such heavy oil difficult. To enhance the flow rate, steam is typically generated on the surface and heated to a temperature of about 500 deg. F. The heated steam is injected into the heavy oil formation. The steam raises the temperature of the heavy oil in the formation thereby reducing its viscosity and allowing the oil to flow more easily towards the tubing of the well where the oil is then brought to the ground surface. The components of the heavy oil obtained typically include not only water and the desired oil but other hydrocarbons and carbon dioxide.
The gases are separated from the oil and water at the ground surface with a separator. The oil is usually the primary product intended to be obtained but the process gases are useful, particularly as a fuel for generating the steam used in the formation injection process.
The composition of the process gas separated from the oil is variable, not only in hydrocarbons, but also in water vapor and carbon dioxide. The energy content of the process gas is of interest since the use of the process gas to generate steam with a known energy content will be more efficient if the amount of air used is optimised and the amount of purchase gas required as a supplementary fuel is minimized. The energy of the process gas is primarily due to the hydrocarbons present as well as any hydrogen sulfide and/or hydrogen. The presence of non-hydrocarbons such as water vapor and carbon dioxide are essentially inert and do not contribute to the energy content of the process. Thus, the non-hydrocarbons reduce the overall energy content of the process gas but contribute to the mass of the process gas used for generating steam.
To assist in the combustion efficiency of the process gas used for generating the steam subsequently used for injection into the producing formation, a controlled flow of air from an external source is added to burn the process gas in the steam generator. The correct ratio of air to the hydrocarbons present in the process gas is desirable for optimum combustion.
The ratio between the hydrocarbons in the process gas and the air required for combustion, each on a volume basis, varies considerably with the composition of individual hydrocarbons in the process gas because the energy content of the individual hydrocarbons varies substantially on a volumetric (MJ/sm3) basis. On a mass basis (MJ/kg), however, the energy content of the total mass of hydrocarbons is substantially independent of the composition of the individual hydrocarbons making up the total mass. By obtaining the total mass of the hydrocarbons, air can be added without concern for the compositions of the individual hydrocarbons in the process gas. The mass flow rate of gases is readily measurable through the use of commercially available Coriolis mass flow meters and the like. The presence of non-hydrocarbons adds deviation to the air to fuel ratio, and the presence of non-hydrocarbons must be accounted for to optimize combustion.
To correctly determine the quantity of air required to be added so as to assist the combustion of the process gas in an optimal manner, the energy content of the process gas has heretofore been measured using a calorimeter. A calorimeter is an on-line instrument that takes a small sample of the process gas and burns it. The energy realised by the combustion of the small sample is measured and that energy quantity is then extrapolated to the process gas as a whole to obtain the overall energy content of the process gas. The air required for that value is then determined.
Calorimeters are expensive and the labor required for the maintenance of such calorimeters adds further expense and may not be readily available. Servicing of calorimeters is therefore a problem. Likewise, where there is a potentially hazardous location where combustion of hydrocarbons is not recommended, the use of calorimeters can be difficult or prohibited. Yet a further disadvantage is that the feed back time for obtaining information from the calorimeter is long. Any adjustment required to the gas and/or input air is which detracts from the early and continuous use of optimal airflow.
The amount of fuel required for the necessary steam generation used for formation injection is dependent on the flow rate of the water to the steam generator, the pressure of the steam generated for the injection, the quality of the steam required and the energy heat content of the process gas. Thus, the energy content of the combination of the process gas and purchase gas where the latter may be used if required, allows the calculation of both the correct air/fuel ratio and the correct fuel/water ratio for the desired steam generation. The energy content of the process gas is desirably obtained quickly to maximize combustion efficiency. It is therefore useful to obtain the mass content of the hydrocarbon, the water and the carbon dioxide within the process gas for optimal steam generation.
The carbon dioxide content of the process gas from hydrocarbon producing wells is known to change relatively slowly so that quickly processing the carbon dioxide content of the process gas is less important that quickly processing the quantity of hydrocarbons and water.
Measuring the total mass flow of gases containing hydrocarbons is known as described above. The use of such mass flow measurement, however, has heretofore been limited to the measurement of gases where hydrocarbons are the only gases present in the gas being analysed. In processing heavy oil using injected steam to enhance oil flow, the presence of non-hydrocarbons such as water vapor and carbon dioxide in significant quantities which accompany the hydrocarbons in the process gas is a unique consideration.